1. Field of the Invention
The present invention relates to a circuit for cancelling a group delay distortion, a frequency characteristic distortion, ghost and others generated in the course of a propagation of a television signal.
2. Description of Related Art
A typical one of conventional ghost cancelling circuits includes an analog/digital converter for receiving a television signal (in general cases, a composite video signal), and a waveform equalizing filter receiving a digitized television signal from the analog/digital converter for equalizing the waveform of the television signal in a frequency characteristics. A ghost cancelling filter block is connected to an output of the equalizing filter, and a digital/analog converter is connected to receive an output of the ghost cancelling filter block, so as to output a compensated analog television signal.
The ghost cancelling filter block mainly comprises an adder having a first input connected to the output of the waveform equalizing filter and a second input connected to receive an artificial ghost signal generated by a transversal filter. An output of the adder is connected to an input of the digital/analog converter. The output of the adder is also connected to an input of an variable delay circuit, whose output is in turn connected to the transversal filter, for the purpose of making a position of the artificial ghost signal generated by the transversal filter consistent with a position of a ghost signal superimposed on the television signal. Thus, as mentioned above, since the artificial ghost signal generated by the transversal filter is added to the television signal including the ghost signal by means of the adder, if the artificial ghost signal generated by the transversal filter has a characteristics in opposition to that of the ghost signal included in the television signal, the ghost signal included in the television signal is cancelled by the artifical ghost signal generated by the transversal filter. Therefore, the television signal having no ghost signal is outputted from the ghost cancelling filter block.
The transversal filter basically includes a number of cascaded delay circuits, a first one of the cascaded delay circuits being connected to receive an input signal, namely, the output of the variable delay circuit. Outputs (called "tap outputs") of all the cascaded delay circuits are in parallel connected to a corresponding number of multipliers, respectively, each of which operates to multiply a received signal by a corresponding so-called tap gain. Outputs of all the multipliers are combined by a summing circuit, so that the artificial ghost signal is generated from the summing circuit.
The above mentioned tap gains of the number corresponding to the number of the multipliers are given by a filter control block, which includes a reference signal detection circuit having an input connected to the output of the analog/digital converter and operating to detect a ghost cancellation reference signal from the television signal. The ghost cancellation reference signal detected and outputted by the reference signal detection circuit is supplied to a Fourier transform block, so that the ghost cancellation reference signal is orthogonally transformed into a frequency domain. The Fourier-transformed ghost cancellation reference signal thus obtained is supplied to a filter coefficient generation block, which also receives a reference waveform outputted from a reference waveform memory. On the basis of a difference between the Fourier-transformed ghost cancellation reference signal and the reference waveform, the filter coefficient generation block generates a filter coefficient for controlling the waveform equalizing filter and recursive coefficients for cancelling the ghost signal.
The filter coefficient and the recursive coefficients thus generated are supplied to an inverse Fourier transform circuit, in which those coefficients are transformed into a time domain. The recursive coefficients outputted from the inverse Fourier transform circuit, which are to be distributed to the tap gain inputs of the transversal filter, are supplied through a gain limiter to a gain controller. The gain limiter operates to restrict tap gains which are not less than 0 dB, to possible maximum values in the range of less than 0 dB. The gain controller receives the tap gains which have been limited below 0 dB, and operates to set the tap gain inputs of the transversal filter to the corresponding received tap gains, respectively. In addition, the gain controller also operates to set the waveform equalizing filter with the filter coefficient outputted from the inverse Fourier transform circuit, and to control a delay time of the variable delay circuit.
Thus, the television signal outputted from the ghost cancellation filter block as a result of the tap setting has a suppressed ghost signal component, and is converted or returned into an analog television signal by the analog/digital converter.
As mentioned above, the conventional ghost cancelling circuit has been such that the limitation of the tap gains outputted from the inverse Fourier transform circuit is only one tap gain limitation. In other words, the tap gains are limited only in a time domain. As a result, the conventional ghost cancelling filter has often oscillated, so that the television signal having passed through the ghost cancelling filter is destroyed. In order to overcome this problem, it is considered to further limit the tap gains outputted from the inverse Fourier transform circuit. However, since the gain limitation in the time domain will limit the gain over a full range of a frequency band, a large gain limitation in the time domain will bring about the result that the ghost signal included in the television signal cannot be completely cancelled.